shader / slab transparency behavior

FORUM -

Been banging my head against this one for a couple of days so I figured it's time to buck up and ask -

I've got a simple shader that creates an alpha-blended circle (soft circle goes from param-defined color in the center to transparent at the edges).

I'm using that shader on a slab and getting the color param just fine but no alpha. I was able to confirm the alpha is calculated properly by testing the shader with calculated alpha on all channels (commented line in the shader below).

I know it's something simple in my implementation that's preventing alpha from being used but I'm at a loss where to look next.

Any help would be appreciated.

Thanks,

N

FRAG SHADER

varying vec4 vertexPosition;
uniform vec4 circleColor;
uniform float intensity;
uniform float circleSizeModifier;

void main()
{
    vec4 center = vec4(0.0,0.0,0.0,0.0);

    vec4 position = vec4(vertexPosition.x, vertexPosition.y * .75, vertexPosition.z, vertexPosition.a);
    float alpha = distance(position.xy, center.xy);
    alpha = alpha * circleSizeModifier;
    alpha = pow(alpha , intensity);

    alpha = pow(alpha ,-1.0);

    gl_FragColor = vec4(circleColor.x, circleColor.y, circleColor.z, alpha * circleColor.a);

//    gl_FragColor = vec4(alpha, alpha, alpha, alpha * circleColor.a);

}

VERT SHADERvarying vec4 vertexPosition;

void main()
{

    gl_Position = ftransform();
    vertexPosition = gl_Position;

}

JXS

TEST PATCH